Currently, liquid crystal displays (LCDs) are becoming a popular display technology for televisions and monitors. LCD panels are made from electronically controlled light valves that require a backlight source in order to produce a visible image. LCD TVs and LCD monitors typically use cold-cathode fluorescent lamps (CCFLs) for this purpose. However, CCFLs have some unique characteristics that must be accounted for when being driven and controlled. For example, CCFLs typically require a DC/AC inverter that transforms a DC voltage signal to an AC voltage signal with an waveform of approximately from 40 KHz to 80 KHz and with an effective voltage of approximately from 500V to 1000V.
The AC waveform needed to drive the CCFLs can be created via the DC/AC inverter which may have several topologies including Royer, full-bridge, half-bridge and push-pull. Both half-bridge and full-bridge topologies require a high-side MOSFET. A drive circuit is essential for such a high-side MOSFET.
FIG. 1 illustrates a prior art DC/AC half-bridge inverter 100. The inverter 100 receives a DC supply voltage VIN and converts it to an AC voltage VOUT. The inverter 100 includes a high-side MOSFET 111 and a low-side MOSFET 113, where the high-side MOSFET 111 is a P-channel MOSFET (P-MOSFET) and the low-side MOSFET 113 is an N-MOSFET. The pair of MOSFETs 111 and 113 are alternatively turned on and off under control of a controller 115. Whereas the low-side MOSFET 113 can be directly driven by the controller 115, a control signal CTLL provided by the controller 115 must be shifted to a higher level through a MOSFET 101, a first resistor 103, a second resistor 105, a first transistor 107 and a second transistor 109 to drive the high-side MOSFET 111. P-MOSFETs have some drawbacks compared with N-MOSFETs. One concern is that P-MOSFETs are less efficient because of their inherent higher on-resistance. Another concern is that P-MOSFETs are more expensive than N-MOSFETs.
FIG. 2 illustrates another prior art half-bridge inverter 200. Repetitive description of similar components contained in FIG. 1 is omitted herein for clarity. In the inverter 200, a high-side MOSFET 201 adopts an N-MOSFET instead of a P-MOSFET to overcome the aforementioned drawbacks. To ensure the high-side MOSFET 201 operates properly, a diode 203 and a capacitor 205 are included as shown in FIG. 2. However, the inverter 200 has a limit on values of the DC supply voltage VIN, which can be observed from the following analysis. Firstly, if a forward voltage of the diode 203 is negligible, a voltage VBC across the capacitor 205 is equivalent to the DC supply voltage VIN. Secondly, it should be appreciated by the skilled in the art that an operating gate-source voltage of the high-side MOSFET 201 is approximately equal to the voltage VBC. Consequently, the gate-source voltage is approximately equal to the DC supply voltage VIN in this instance. Given that most of MOSFETs define gate-source voltages not to exceed 20V and overcharging MOSFETs can lead to “shoot-through” where both the high-side and low-side MOSFETs switch on, in the inverter 200, there is a limit on values of the DC supply voltage VIN to prevent occurrence of “shoot- through”.
Thus, it is desirous to have an inverter which adopts N-MOSFETs and at the same time operates in a wide DC supply range. It is to such an inverter that the present invention is primarily directed to.